In view of the recent instability of the price of crude oil and natural gas, there has been renewed interest in alternate sources of energy and hydrocarbons. Much of this interest has been centered on recovering hydrocarbons from solid hydrocarbon material such as oil shale, coal, and tar sands by pyrolysis or upon gasification to convert the solid hydrocarbon-containing material into more readily usable gaseous and liquid hydrocarbons.
Vast reserves of hydrocarbons in the form of oil shales exist throughout the United States. The Green River formation of Colorado, Utah, and Wyoming is a particularly rich deposit and includes an area in excess of 16,000 square miles. It has been estimated that an equivalent of 7 trillion barrels of oil are contained in oil shale deposits in the United States, almost sixty percent located in the Green River oil shale deposits. The remainder is largely contained in the leaner Devonian-Mississippi black shale deposits which underlie most of the eastern part of the United States.
Oil shales are sedimentary inorganic materials that contain appreciable organic material in the form of high molecular weight polymers. The inorganic part of the oil shale is marlstone-type sedimentary rock. Most of the organic material is present as kerogen, a solid, high molecular weight, three-dimensional polymer which has limited solubility in ordinary solvents, and therefore cannot be readily recovered by simple extraction.
A typical Green River oil shale is comprised of approximately 85 percent mineral components, of which carbonates are the predominate species, and lesser amounts of feldspars, quartz, and clays are also present. The kerogen component represents essentially all of the organic material. A typical elemental analysis of Green River oil shale kerogen is approximately 78 weight percent carbon, 10 weight percent hydrogen, 2 weight percent nitrogen, 1 weight percent sulfur, and 9 weight percent oxygen.
Most of the methods for recovering kerogen from oil shale involves mining the oil shale, crushing it, and thermally decomposing (retorting) the crushed oil shale. In view of the fact that approximately 85 weight percent of the oil shale is mineral components, unless something is done to remove these minerals, most of the material which is fed, heated up, and circulated in a retort cannot produce oil. This high percentage of inorganic material significantly interferes with subsequent shale processing to recover the kerogen. For example, in retorting oil shale, either large or numerous retorts are needed to process the commercial quantities involved. Moreover, a substantial amount of heat is expended and lost in heating up the inorganic minerals to retorting temperatures and cooling them back down again.
Another problem associated with the presence of large amount of inorganic mineral matter in the oil shale is pollution. In the retorting process, contaminating fines are produced, and therefore must be disposed of. The greater the quantity of minerals, the greater the quantity of fines. Another source of pollution is the spent shale recovered from the retort. During retorting, chemical reactions occur in the shale as the kerogen is volatilized. This results in a residue of chemical compounds in the spent shale leaving the retort. These compounds can present a hazard in surface water pollution after they have been discarded.
As a result of problems associated with the high percentage of minerals in oil shale, it can be economically beneficial to reject the minerals prior to retorting. The process of rejecting these minerals and concentrating the kerogen prior to retorting is called "shale beneficiation." This beneficiation is basically divided into two steps. The first step is liberating the kerogen from the mineral matter. The second step is separating the kerogen from the mineral matter.
An essential part of liberating the kerogen from the mineral matter is comminuting the shale. There are many options for comminuting the shale. Hazemag mills, semiautogenous (SAG) mills, ball mills, and tower mills can be effective for various stages of comminuting. The number of comminution stages and the selection of the most efficient mill depends upon the intrinsic grain size of the kerogen and the extent of kerogen liberation required.
In a SAG mill, which is a cascade mill in which about 10 volume percent steel balls supplement the oil shale solid feed as comminution media, the shale can be comminuted down to about 1/2 in. top size. A ball mill, which is a tumbling mill using about 50 volume percent steel balls as comminution media, can comminute the shale down to about 0.003 in. top size. To obtain a top size of less than 0.003 in., a tower mill can be used. A tower mill is a stirred ball mill that uses attrition as the mechanism for size reduction.
After comminuting the shale to produce kerogen-rich particles and mineral-rich particles, the second step of beneficiation is separating these particles from each other. The two basic types of kerogen-rich/mineral-rich particle separation are chemical and physical separation.
Chemical separation includes leaching of minerals, such as acid leaching of carbonates, or extraction of kerogen by chemically breaking the kerogen bonds. U.S. Pat. Nos. 4,176,042 and 4,668,380 disclose examples of chemical beneficiation of oil shales.
One type of physical separation is density separation. Density separation is possible because kerogen has a specific gravity of about 1 gm/cm.sup.3 and because min eral components in shale have a density of about 2.8 gm/cm.sup.3. Heavy media cyclone is a process for separating by density relatively coarse oil shale particles. An example of a heavy media separation method is disclosed in U.S. Pat. No. 4,528,090. In general, the aim of heavy media separation is to separate oil shale into a kerogen rich fraction having low density and a kerogen-lean fraction having high density. The liquid medium used is a mixture of water and finely ground magnetite and ferrosilicon. By varying the concentration of the magnetite and ferrosilicon, the medium can be made to have a density from 1.8 to 2.4 gm/cm.sup.3 so that the shale can be split at the density required. The kerogen-rich material floats and is taken overhead and the kerogen-lean material goes into the underflow from the cyclone. The disadvantages of this process are that it relies upon an inherent natural heterogeneity among oil shale particles and it has not been successful in separating small particles.
Another type of physical separation is surface property separation. An example of surface property separation is froth flotation. In this process, oil shale particles are mixed with an aerated aqueous solution. Since the kerogen-rich particles have greater hydrophobic character than mineral-rich particles, the kerogen-rich particles preferably adsorb onto air bubbles, thereby causing the kerogen-rich particles to float. Subsequently, the froth containing these kerogen-rich particles is removed. Additives can be used to improve kerogen grade and recovery. One disadvantage of the froth flotation process is the oil shale particles are required to be comminuted to a fine particle size prior to froth flotation. Another disadvantage of froth flotation is that the effects of different types of collectors, frothers, and dispersants are difficult to predict. In addition, floated, kerogen-enriched shale has a tendency to have a higher concentration of carbonates than starting shale. An example of a froth flotation process is disclosed in U.S. Pat. No. 4,673,133.
Another example of surface property separation is kerogen agglomeration. Kerogen agglomeration is a process whereby shale is comminuted or kneaded in the presence of an organic liquid and water to form large agglomerates of the kerogen-rich particles, while small mineral-rich particles disperse into the water phase.
In Reisberg, J., "Beneficiation of Green River Shale by Pelletization," American Chemical Society (ASCMC8), V. 163 (Oil Shale, Tar Sands, and Related Materials), pp. 165-166, 1981, ISSN 00976156, a form of kerogen agglomeration of shale is disclosed. This reference describes precomminuting the oil shale to a size small enough to pass through a 0.0059 in. (100 mesh) screen. This shale is subsequently comminuted in the presence of heptane and water to form a kerogen-enriched fraction in the form of discrete flakes or pellets and mineral-rich particles dispersed in an aqueous phase. These pellets are then separated from the aqueous phase using sieves. The major disadvantage of the process disclosed by this reference is the comminution cost associated with the initial comminution of the shale is prohibitively high and requires an excessive power outlay. An estimated total comminution power input for this process is 130 Kw-hr/ton of shale.
During kerogen agglomeration of the oil shale, the carbonate level increases along with the organic carbon concentration in the beneficiate. The presence of the carbonates can make oil shale more difficult to beneficiate. It is known that the kerogen-rich agglomerates produced during kerogen agglomeration retain or concentrate the calcium and magnesium carbonates minerals.
One way to avoid this problem is to remove the carbonates before physical separation. In Smith, J. W.; L. W. Higby "Preparation of Organic Concentrate from Green River Oil Shale," Analytical Chemistry, Vol. 32, No. 12, November 1960, the carbonate problem was address by removing the carbonates prior to physical separation by first contacting the oil shale with a 5 percent acetic acid solution. One difference between the method disclosed in the Smith reference and the instant invention is in the Smith method the oil shale is acid-treated prior to physical separation. In the instant invention, the shale is acid-treated after kerogen agglomeration. Another way of describing this difference is, in the Smith reference, the acid solution is contacted with raw oil shale particles having a kerogen concentration of about 6-30 weight percent whereas, in the instant invention, an acid solution is contacted with a kerogen-rich oil shale agglomerate. This agglomerate has a kerogen concentration of about double that of raw oil shale particles (the exact kerogen concentration of kerogen-rich oil shale agglomerates will depend on the kerogen weight percent of the raw oil shale, the mineral composition of the raw oil shale, and the type of process used to agglomerate the kerogen contained within the oil shale). Although the Smith method may be useful for obtaining kerogen for analytical studies, it would not be practical for commercial applications because of the cost of using a large amount of acid.
In U.S. Pat. No. 4,584,088, there is disclosed acid-treating a shale that has previously been treated chemically to aid in beneficiation. In this method, raw oil shale is first contacted with an aqueous caustic solution to produce a shale product of substantially transformed mineral content. Then the shale product is separated. Next the separated shale product is acid-treated treated. This method acid treats shale that has already been chemically beneficiated. One difference between this method and the instant invention is the instant invention acid treats physically beneficiated shale, whereas the method disclosed in U.S. Pat. No. 4,584,088 acid-treats chemically beneficiated shale.
There is a need for a viable, cost effective process for removing carbonates from kerogen-agglomerated oil shale.